The invention relates to a transport device for transporting an endless form to and from the printing roller which is journalled in walls of the frame of the printing mechanism of an office machine, the transport device comprising transport chains which cooperate pairwise and which are arranged on at least one drive shaft which is journalled in sidewalls, and also comprising a drive which transmits the rotary movement of the printing roller to the drive shaft.
U.S. Pat. No. 3,161,277 discloses a device for permanently holding taut an endless form which is transported to and from a printing roller by a pair of transport chains. A shaft is arranged near the printing roller across the full width of the endless form in order to keep the endless form taut between the transport chains and the printing roller. In order to keep the form taut also in the deflected position of the printing roller, the shaft is journalled so that it can be lowered. Therefore, on the side of the frame of the printing mechanism there are provided sliding rails which are shifted by an eccentric. Moreover, the sliding rails comprise slits in which a pin of the mounting of the printing roller engages. The lifting and lowering of the shaft are thus frictionally coupled to the pivoting of the printing roller.
The known device has the drawback that the rigid coupling means that the distance between the transport chains and the printing roller always remains constant during operation, so that the inserted endless forms are subject to the same tension during transport only when they have the same thickness.
It is known, however, that different endless forms are inserted into office machines. For example, the paper thickness of the individual layers as well as the number of layers may be different. As a result, transport perforations of the endless form are liable to be damaged, notably when it consists of only one layer of paper. In the case of several layers, the endless form, moreover, is constantly subject to uncontrollable tensions due to manufacturing tolerances of the printing roller and the transport chains as well as tolerances in the distance therebetween.